Chromatic dispersion is one of the major sources for signal distortions in high-speed optical communications. For example, OC192 systems (10 Gbit/s Non-Return-to-Zero format) are limited about 70 km for conventional single mode fibers (SMF-28) without compensation for chromatic dispersion. Among a variety of techniques for dispersion compensation, dispersion compensating fibers (DCF) and fiber Bragg gratings (FBG) are the most techniques used in practical applications.
As the demand for bandwidth keeps increasing, the requirements for dispersion compensation also increases. The most efficient way to increase system capacity is to increase the number of channels using the wavelength-division-multiplexing (WDM) technology. However, the existence of higher-order chromatic dispersion in optical fibers makes it difficult to provide chromatic dispersion compensation for all of the channels. The dominating effect of higher-order dispersion is the third-order dispersion, which is also known as the dispersion slope. In other words, dispersion slope describes the different chromatic dispersion that each WDM channel experiences. The broader the optical bandwidth that the WDM channels occupy, or, the longer the transmission distances, the greater the effect of dispersion slope. Therefore, compensation for dispersion slope has become crucial for high capacity WDM systems.
Different types of optical fibers have different dispersion characteristics. In other words, different dispersion slope compensations are required for different fibers. For example, dispersion compensating fibers (DCFs) are considered one of the most reliable techniques for compensating for both dispersion and dispersion slope for the single mode fiber SMF-28. However, it is difficult to design a suitable DCF for dispersion-shifted fibers (DSF) due to the limitations of the optical fiber design. Ideally, fiber Bragg gratings (FBG) are preferable over DCFs for several attractive reasons such as, virtually no optical nonlinearity, low insertion loss, compact size, and flexibility for different fiber types. However, a group-delay ripple associated with an FBG makes it inferior in most applications when compared to a DCF. Therefore, it becomes desirable to provide a technique to compensate for a large amount of chromatic dispersion. Unfortunately, the group-delay ripple also becomes larger for larger amount of chromatic dispersion. Unless the group delay ripple of the FBG can be made small enough, the application of FBGs in dispersion compensation is limited.
It is desirable to provide a technique for higher-order chromatic dispersion and dispersion slope compensation for virtually any type of optical fibers used in high-speed optical communication systems without suffering severe degradation due to a group-delay ripple of fiber Bragg gratings.